function demo(id) {
  let ele = document.getElementById(id);
  let overflow = ele.style.overflow;
  ele.style.overflow = overflow === "hidden" ? "visible" : "hidden";
}

// binding
var b1 = document.getElementById("b1");
b1.addEventListener("click", ()=>{demo("bfc1")});

var b2 = document.getElementById("b2");
b2.addEventListener("click", ()=>{demo("bfc2")});

var b3 = document.getElementById("b3");
b3.addEventListener("click", ()=>{demo("bfc3")});

var b4 = document.getElementById("b4");
b4.addEventListener("click", ()=>{demo("bfc4")});